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Jul 10, 2010

July 5, 2010
Courtesy of the Eu­ropean Space Agen­cy
and World Science staff

In a rare event next Thurs­day, some sky­watch­ers in Eu­rope will be able to see a star briefly van­ish as an as­ter­oid pas­ses be­tween it and us. As­tro­no­mers say the event may be the only eclipse of a star by an as­ter­oid this cen­tu­ry vis­ible with the un­aided eye.

Eve­ry­one is fa­mil­iar with a so­lar eclipse, when our Moon passes in front of the Sun and blocks its light for sev­er­al min­utes.

Star chart showing the sky from central Europe, facing south just be­fore mid­night on July 8. The star Delta Oph­iuchi will be about half-way be­tween the hori­zon and the ze­nith or "top" of the sky. (Cre­dits: ESA, created with Guide 8 by Pro­ject Plu­to, http://www.projectpluto.com)

A si­m­i­lar situa­t­ion can hap­pen with as­ter­oids, the Sun-or­b­it­ing, rocky or me­tal­lic ob­jects left over from the forma­t­ion of the So­lar Sys­tem or formed by crashes be­tween oth­er as­ter­oids.

We know of about 400,000 of these dark bod­ies, which range in size from a few hun­dred kilo­me­tres (miles) to just a few me­tres (yards). Smaller ones are hard to de­tect.

While an as­ter­oid is far too small to co­ver the Sun, one will oc­ca­sion­ally move di­rectly in front of one of the stars in the night sky and block its light from us, caus­ing a stel­lar eclipse or oc­culta­t­ion. Since as­ter­oids move fast, these events typ­ic­ally last just a few sec­onds.

Nor­mally the oc­culted star is so faint the event can only be seen via tel­e­scope. But the night of 8-9 Ju­ly, a star vis­i­ble to the na­ked eye, Del­ta Ophi­uchi (the fourth-brightest star in the con­stella­t­ion Ophi­uchi), will be oc­culted by as­ter­oid Ro­ma, about 50 km (30 miles) wide.

This means the eclipse will be vis­i­ble only along a path of about that same width, cross­ing cen­tral Eu­rope, Spain and the Ca­nary Is­lands. At about 11:57 p.m. Cen­tral Eu­ropean Sum­mer Time, ob­servers on a line run­ning be­tween Stock­holm - Copen­hagen - Bre­men - Nantes - Bil­bao will see the star dis­ap­pear for about five sec­onds as its light is blocked by the as­ter­oid.

Since most as­ter­oids are too small to be re­solved with ground-based tel­e­scopes, as­ter­oid oc­culta­t­ions are the only di­rect way of meas­ur­ing the size of such an ob­ject. When sev­er­al ob­servers rec­ord such an event, us­ing vi­deocam­er­as with pre­cise tim­ing, the times when they see the oc­culta­t­ion help to meas­ure the shape of the as­ter­oid.

Since we know the speed of the as­ter­oid, the dura­t­ion of the oc­culta­t­ion can be con­vert­ed di­rectly to a length. This al­lows sci­en­tists to re­con­struct the size and shape of the ob­ject.

As­ter­oids com­ing close to Earth are the fo­cus of a new Eu­ropean Space Agen­cy mon­i­tor­ing pro­gram, known as the Near Earth Ob­ject seg­ment of the Space Situa­t­ional Aware­ness Pre­par­a­to­r Pro­gramme.